For the continuous determination of filling levels of liquids or bulk materials in vessels, sensors working, for example, according to the radar principle may be employed. With this method the delay is measured that it takes for the microwaves transmitted by the sensor to cover the distance from the sensor to the filling matter surface and back. The microwaves used, which, for example, may be in a frequency range of between about 1 and 100 GHz, are usually transmitted by an antenna in the form of electromagnetic waves. Then, after reflection by the filling level surface, they are received again by the sensor.
Devices are also known which guide a wave along a wave guide from a sensor to the surface of a filling matter and receive the wave reflected on the filling matter surface using the same path. Both with this method and with methods employing the radar principle, the reflection of the waves on the filling matter surface is due to the change in the propagation impedance of the wave at this point.
For the determination of the desired wave delay, there are various radar principles. One of these is the impulse delay method (pulse radar method), another is the frequency modulated continuous wave (FMCW) radar method. In the FMCW radar method, the delay is determined in an indirect manner by transmitting a frequency modulated signal and creating a difference between the transmitted and the received momentary frequency.
The pulse radar method, on the other hand, uses the radiation of short microwave pulses, also known as bursts, wherein the direct time duration is determined between the transmission and the reception of the individual pulses. From this duration it is then possible to derive the distance between the sensor and the filling matter surface and thus, since the structural position of the sensor in the vessel and the vessel dimensions are known, the vessel filling level.
Usually radar filling level measuring devices transmit linearly polarized microwaves. Linear polarization means that the electrical field intensity vector of the wave is always in the same plane extending along the propagation direction of the wave. The polarization direction of the transmitted wave is therein determined by the structure of the transmitting antenna. If for example a linearly polarized wave is reflected by a filling matter, the polarization direction of the linearly polarized wave does not usually change. If anything, the wave experiences a phase jump of 180°, or λ/2, which causes the field intensity vector of the wave to be reversed on reflection.
In any case, when a linearly polarized wave is reflected, its polarization direction is maintained, which results in the reflected wave being able to be received by a receiving antenna similar to the transmission antenna. As a result, there need not be separate transmitting and receiving antennas, but instead a single antenna may be used capable of transmitting and receiving microwaves at the same time. Such sensors are also known as monostatic radar systems, characterized on the one hand by their ability to dramatically reduce costs, since it is not necessary to provide two different antennas—one for transmitting and one for receiving. On the other hand, such monostatic radar systems have a relatively small structural size so that there is no unnecessary waste of space, which is usually limited by the existing vessel openings for mounting the filling level sensor.
With the simultaneous use of a single antenna for both transmitting and receiving, it is, however, necessary to separate the electrical transmitting signals sent from the microwave generator to the antenna and the receiving signals sent back from the antenna to the microwave receiver. This is the only way to ensure that the transmitting signals from the microwave generator do not directly reach the microwave receiver without ever having been transmitted by the antenna.
Usually this transmitting/receiving separation is carried out by sensors radiating linearly polarized waves, using, for example, a circulator or a directional coupler. These circuit components are connected to each of the microwave generator, the microwave receiver and the antenna, using a single lead, ensuring on the one hand that the signals generated by the microwave generator are essentially sent to the antenna and not directly to the microwave receiver. On the other hand, the reflection signals coming back from the antenna are essentially forwarded to the microwave receiver and not to the microwave generator.
A radar filling level measuring device having a combined transmitting and receiving antenna and using linear polarization can be particularly advantageous in that, in addition to the elimination of a second antenna, a single lead to the antenna, for example, in the form of a coaxial cable having an associated antenna feeding system, is sufficient. This results in a simple structure of the antenna assembly. However, circulators are usually relatively expensive. Much more problematic than the costs associated with these systems, however, are the signal losses resulting from the signal attenuating properties in particular of the directional coupler, which cause the signal-to-noise ratio of the measuring system to be reduced due to signal loss.
Another approach for realizing a transmitting/receiving antenna is the use of an elliptically polarized wave or, as a special case, a circularly polarized wave, instead of a linearly polarized wave. With an elliptically polarized wave, the electromagnetic field intensity vector spirals along the propagation direction in a helix with an elliptical cross section, whereby the field intensity vector results from superimposing two wave components having different amplitudes. In the special case of a circularly polarized wave, the electromagnetic field intensity vector also spirals along the propagation direction. The spiral, however, has a circular cross section and the two wave components combining to form the resulting wave have equal amplitudes. Depending on the sense of rotation of the spiralling, a distinction can be made between clockwise and anticlockwise (i.e., counter-clockwise) circular polarization.
Such elliptical or circular polarization is suitable in particular for radar filling level measurement in tall, relatively narrow vessels since in these applications part of the wave radiated from the antenna does not directly reach the filling matter surface but, due to the non-ideal directional characteristic of the antenna, is laterally deflected by the vessel wall, before it is reflected on the filling matter and reflected back to the antenna causing such indirectly reflected signal components to have slightly longer delays than a signal reflected back directly from the filling matter. Due to the limited local resolution of the systems envisaged here, two echoes having only a small delay difference overlap, which in traditional sensors has the result that the filling matter echo to be evaluated is superimposed with, and interfered by, echoes deflected by the vessel wall, thereby reducing the measuring precision and measuring reliability of the sensor.
This drawback can be solved by radar filling level measuring devices using circularly polarized waves, since such sensors distinguish between various reflection components and may suppress undesirable components such as those deflected by the vessel wall. The reason for this is that a circularly polarized wave changes its sense of rotation so that a twice reflected wave has a sense of rotation opposite to that of a once reflected wave. Since, however, the receiving antenna only receives waves having one sense of rotation, the interfering reflections having an opposite sense of rotation do not reach the microwave receiver and therefore cannot have a negative effect on the measurements.
Furthermore, the use of the circular polarization may be useful for the determination of the filling level of a bulk material in a vessel when the bulk material has an irregular and fissured surface. Linearly polarized waves are reflected from such bulk material surfaces in differing degrees depending on the position of the polarization plane with respect to the surface structure which, in unfavourable bulk material conditions, may even result in a situation where the correctly reflected signal component is so small that a reliable measurement may no longer be guaranteed. If, however, a circularly polarized wave is used in such a case, a reflection may result whose multitude of different components averages out resulting in a reflection of average intensity, which, however, is definitely stronger than that achieved with the use of a simple linearly polarized wave.
A drawback of the prior art radar filling level measuring devices having circular polarization is that to date the generation of a circularly polarized wave has been relatively expensive. It is known, for example, from EP 0 592 584 filed Jun. 26, 1992, that by inserting a dielectric disc within a hollow conductor a wave can be generated with a circular polarization from a linearly polarized wave. However, this additional hollow conductor section in which the dielectric disc has been inserted requires considerable space. Moreover, the practical realization of this approach, only suggested in principle in the document mentioned, requires additional structural measures such as the protection of the hollow conductor cavities against soiling, supporting the structure against high vessel pressures or for the chemical resistance of the assembly, so that this apparatus is of little practical use.